Dimethyl ether in its ground state, v = 0, and lowest two torsionally excited states, v 11 = 1 and v 15 = 1, in the high-mass star-forming region G327.3-0.6
Context. One of the big questions in astrochemistry is whether complex organic molecules are formed in the gas phase after evaporation of the icy mantles of interstellar dust grains or at intermediate temperatures within these icy mantles. Dimethyl ether (CH3OCH3) is one of these species that may form through either of these mechanisms, but it is yet unclear which is dominant.
Aims. The goal of this paper is to determine the respective importance of solid state vs. gas phase reactions for the formation of dimethyl ether. This is done by a detailed analysis of the excitation properties of the ground state and the torsionally excited states, ν11 = 1 and ν15 = 1, toward the high-mass star-forming region G327.3-0.6. Methods. With the Atacama Pathfinder EXperiment 12 m submillimeter telescope, we performed a spectral line survey toward G327.3-0.6 around 1.3, 1.0, and 0.9 mm as well as at 0.43 and 0.37 mm. The observed CH3OCH3 spectrum is modeled assuming local thermal equilibrium.
Results. CH3OCH3 has been detected in the ground state, ν= 0, and in the torsionally excited states ν11 = 1 and ν15 = 1, for which lines have been detected here for the first time. The emission is modeled with an isothermal source structure as well as with a non-uniform spherical structure. In the isothermal case two components at 80 and 100 K are needed to reproduce the dimethyl ether emission, whereas an abundance jump at 85 K or a model with two abundance jumps at 70 and 100 K fit the emission equally well for the non-uniform source model. The emission from the torsionally excited states, ν11 = 1 and ν15 = 1, is very well fit by the same model as the ground state.
Conclusions. For non-uniform source models one abundance jump for dimethyl ether is sufficient to fit the emission, but two components are needed for the isothermal models. This suggests that dimethyl ether is present in an extended region of the envelope and traces a non-uniform density and temperature structure. Both types of models furthermore suggest that most dimethyl ether is present in gas that is warmer than 100 K, but a smaller fraction of 5%-28% is present at temperatures between 70 and 100 K. The dimethyl ether present in this cooler gas is likely formed in the solid state, while gas phase formation probably is dominant above 100 K. Finally, the ν11 = 1 and ν15 = 1 torsionally excited states are easily excited under the density and temperature conditions in G327.3-0.6 and will thus very likely be detectable in other hot cores as well.